Heat treated u-nb alloys



Nov. 24, 1959 R. K. MCGEARY EIAL 2,914,433

mu TREATED U-Nb ALLOYS Filed Oct. 11, 1955 1 I- 6 I I I b 0 :w I I N 3 Iv! z 7 I b L a N m 2 z 1 m 8 7 B 1 x n O O O O D O O O m m w 5 o EPEQEPP Murray i Weight Loss Gamma Quenched Uranium-Niobium Alloys in 500F. Water.

Test Time, Days United rates 2,914,435 HEAT TREATED U-Nb ALLOYS RobertK. 'McGeary, Pittsburgh, Pa., and William M.

This invention relates to binary alloys of niobium and uranium andmembers prepared therefrom, particularly for use as fuel elements innuclear reactors.

The use of uranium members as fuel elements'in certain types of nuclearreactors employing water as a moderating medium and for other purposesresults in unsatisfactory operation because of the high rate ofcorrosion of the uranium when in contact with hot water. In many cases,uranium members of substantial thickness will disintegrate completely inless than one day when in contact with hot Water at a temperature of 600F. While it has been proposed to apply a protective cladding material tothe uranium fuel elements, there are substantial possibilities forvoids, fissures, cracks and the like to be present which will permit thehot water or steam to reach the uranium and thereby cause corrosion,swelling and other undesirable results which may cause improper functionof the reactor.

While it has been proposed to alloy uranium with various metals in orderto reduce corrosion resistance when in contact with water at elevatedtemperatures, it has been discovered that many alloying elements do notimprove the corrosion resistance, and, in some cases, they may evenaccelerate corrosion.

The object of this invention is to provide members suitable for use asfuel elements in a nuclear reactor comprising an alloy composed of from7% to 20% by weight of niobium and the balance being uranium, eithernatural or enriched, the alloy being substantially entirely in the gammaphase.

Other objects of the invention will, in part, be obvious and will, inpart, appear hereinafter. For a better understanding of the nature andobjects of the invention, reference should be had to the followingdetailed description and drawing, in which:

, Figure l is a phase diagram of uranium-niobium alloys; and

Fi'g. 2 is a graph plotting corrosion resistance for three differenturanium-niobium alloys in 500 F. water.

Referring to Fig. l of the drawing, there is illustrated a phase diagramof the alloys of niobium and uranium. It will be noted that the solidalloys at a temperature of about l000 C. are essentially all of thegamma phase structure. However, on being cooled to a temperature ofbelow 625 C., the alloys tend to transform into the alpha phase ofuranium in which niobium has substantially no solid solubility, the rateof transformation depending on the niobium content. The alpha uranium ischaracterized by poor corrosion resistance when in contact with hotWater, for example, at 600 F. The presatent "Fee 2,914,433 Patented Nov.24, 1959 2 ence of particles of undissolved niobium in alpha uraniumcenters no improved corrosion resistance and may cause acceleratedcorrosion.

We have found that alloys of uranium containing from 7% to 20% by weightof niobium can be heated to a temperature in the gamma field of thephase diagram and quenched from this temperature, for instanceapproximately 900 C;, at which they are essentially all of the gammaphase, to room temperature where they will retain the gamma structure.Alloys having 6% and less of niobium transform rapidly from the gamma tothe alpha phase. Such transformation in the low niobium content alloysis initiated rapidly and complete transformation occurs more rapidly asthe alloy is heated to temperature s 'above 300 C. By contrast, alloyscontaining 7% to 20% of niobium in the gamma phase have been heated formany days at temperatures of up to 650 F. (343 C.) without showing anyappreciable amount of transformation to the alpha phase.

Member's prepared from alloys composed of from 7% to 20% niobium and thebalance being uranium, when quenched from a temperature of approximately900 C. to room temperature to retain the gamma phase, exhibit highcorrosion resistance to hot water at elevated tem p'erat'u'res. Weprepared a series of binary uranium alloys comprising (a) 9% by weightof niobium, (b) Il /2% by Weight ofniobium, and (c) 20% by weight ofniobium, which alloys had been heated for 24 hours at 900 C. and thenwater quenched to room temperature. Specimens of these alloys weretested in an autoclave filled with hot water at temperatures of 500 F.,575 F. and 650 F. Our tests showed that the average corrosion rate forall the specimens in 500 F. water was 0.008 milligram per squarecentimeter per hour. In 575 F. water, the average corrosion rate was0.02 milligram per square centimeter per hour. When tested in 650 F.water, the average corrosion rate of the three alloys was 0.045milligram per square centimeter per hour. By comparison,alloyscontaining from 2% to 4% niobium prepared by annealing and quenchingunder the same conditions disintegrated completely in a matter of hoursin hot water under each of these test conditions.

The alloys of the present invention may be prepared by melting thedesired proportions of niobium and uraniur'n in an induction furnacecomprising a graphite crucibie coated with a wash of zirconium oxide.The uranium may comprise natural uranium containing not over a fewhundredths of 1% of impurities. The uranium may be entirely naturaluranium or natural uranium that has been enriched with, for example, 10%of uranium 235. The molten alloy is poured into a graphite crucible toproduce an ingot. In some instances the melt may be cast within aprecision mold into the desired final form of member. We have found itordinarily preferable to hot work the cast ingot by forging, extrusion,rolling and the like into the desired shape of member. The ingot may beheated in a salt bath to a temperature of from 1700 to 2l0O F., and willthen be readily extrudable into strips, bars or other desiredstructures. The alloy may be hot forged when heated to temperatures of1700 F. In some instances, we have secured ingots of improvedhomogeneity and structure by employing the original cast ingot as theconsumable electrode in an arc The arc furnace may be operated as setforth furnace.

in copending application, Serial No. 367,524, assigned to the assigneeof the present invention. The are cast ingot may be readily extruded,rolled or forged to produce members of suitable size and shape.

The following examples are illustrative of the practice of theinvention:

Example I Into a crucible of an induction furnace there was placed 9parts by weight of niobium and 91 parts by weight of natural uraniumhaving the following impurities:

Other elements Less than 2 The alloy after having been melted was thenpoured into a graphite mold and cast into an ingot of a diameter of 2.4inches. The ingot was then extruded into a bar. Specimens of a thicknessof inch having a surface area of 5 square centimeters were cut from theextrusion. The members were homogenized by annealing at 900 C. for 24hours and quenched in water to retain the gamma phase throughout. Thespecimens were placed in an autoclave containing water at 500 F. andtested for a prolonged period of time of 275 days. For the first 150days, the total corrosion on the specimens comprised 22 milligrams persquare centimeter. The corrosion rate increased thereafter so that atthe end of 275 days the total weight loss comprised 135 milligrams persquare centimeter.

Example 11 Members comprising Il /2% by weight of niobium and thebalance being natural uranium were prepared by the process of Example I.The annealed and quenched specimens having substantially all gamma phasewere placed in an autoclave and tested in contact with water at 500 F.for 300 days. The total corrosion of the specimens at the end of 275days was approximately 88 milligrams per square centimeter which wassomewhat less than that of the alloy of Example I. However, the initialcorrosion rate was somewhat greater for the alloy of this example thanwas exhibited by the 9% niobium alloy.

Example III Specimens were prepared from a 6% niobium-uranium alloyfollowing the procedure of Example I. The specimens of the alloy wereplaced in an autoclave containing water at 500 F. The specimens werefound to corrode at a much higher rate than the specimens of Examples Iand II. Thus at the end of 75 days, the corrosion had caused a weightloss of 155 milligrams per square centimeter and complete disintegrationand failure occurred soon thereafter.

The data obtained in the tests set forth under Examples I and II, alongwith the 6% niobium specimens. are plotted in Fig. 2 of the'drawing. Itwill be observed that the niobium alloys of the present invention have amuch greater resistance to corrosion as exemplified by the weight lossin hot water than alloys containing 6% of niobium. Alloys of uraniumcontaining less than 6% niobium would have been completely disintegratedin a matter of a few days at most. y

We have discovered outstanding corrosion resistance is exhibited byuranium alloys containing from, 8% to 12% by weight of niobium.

For use as fuel elements in nuclear reactors, it is ordinarily desirableto employ alloys containing the maximum proportion of uranium and theminimum amount of the alloying element, such as niobium. Consequently,alloys containing 8% to 12% of niobium will be preferred to the alloyscontaining, for example, 18% to 20% niobium.

We have secured good results with respect to corrosion resistance fromalloys comprising 16% niobium and 20% niobium, the balance being naturaluranium in each case. The gamma quenched alloys when placed in contactwith water at 650 F. withstood more than 56 days in each case beforecomplete corrosion failure took place.

It is desirable in all cases in preparing members from alloys comprisingfrom 7% to, 20% by weight of niobium, the balance being uranium, tohomogenize the members at temperatures in the gamma field, ordinarily weuse temperatures above 900 C., for at least several hours and to quenchthe members to retain the gamma phase at room temperature and underconditions in reactors. Water quenching has given good results. Coolingwith inert gas or other means to reduce the temperature of the alloyfrom 900 C. to room temperature in an hour or two will also maintain thegamma phase.

It will be understood that the alloys of the present invention may beclad with protective coatings of materials such, for example, as thezirconium alloy set forth in copending application, Serial No. 416,396,now Patent No. 2,772,964 assigned to the assignee of the presentinvention.

It will be understood that fuel elements prepared from the alloys of thepresent invention when employed in nuclear reactors embodying water forcooling, moderating and for extracting the heat therefrom, should beoperated at temperatures such that the fuel elements do not exceed atemperature of approximately 350 C. for any appreciable length of time.

Upon being tested by irradiation, specimens of the gamma phase alloys ofthis invention showed quite small isotropic changes in shape as comparedto low niobium alloys which changed in shape irregularly and exhibitedpronounced dimensional changes in certain directions.

It will be understood that the above description and drawing are onlyexemplary and not in limitation of the invention.

We claim as our invention:

1. A member suitable for use as a fuel element in a nuclear reactorcomprising an alloy wrought to the shape of the member, the alloycomposed of from 7% to 20% by weight of niobium and the balance beinguranium, the alloy being substantially entirely in the gamma phase, thealloy having been produced by working an ingot of the alloy into themember, homogenizing the member by annealing it at a temperature in thegamma phase field and quenching the member to retain the gamma phasestructure of the alloy.

2. A member suitable for use as a fuel element in a nuclear reactorcomprising an alloy wrought to the shape of the member, the alloycomposed of from 8% to 12% by weight of niobium and the balance beinguranium, the alloy being substantially entirely in the gamma phase, thealloy having been produced by working an ingot of the alloy into themember, homogenizing the member by annealing it at a temperature in thegamma phase field and quenching the member to retain the gamma phasestructure of the alloy.

3. In the method of preparing a member suitable for use as a fuelelement in a nuclear reactor comprising an alloy composed of from 7% to20% by weight of niobium and the balance being uranium, thesteps'comprising working an ingot of the alloy into the member,homogenizing the member by annealing at a temperature in the gamma phasefield and quenching the member to retain the gamma phase structure ofthe alloy.

(References on following page) References Cited in the file of thispatent UNITED STATES PATENTS 2,830,896 Seybolt Apr. 15, 1958 OTHERREFERENCES Saller et a1.: Compilation of US. and UK. Uranium and ThoriumConstitutoral Diagrams, publ. June 1, 1955, by U.S.A.E.C. as BMI-1000,entire publication 141 6 pages, pages 48 and 49 relied upon. Availablefrom O.T.S., Dept. of Commerce, Washington 25, DO. ($0.90).

Jones: WAPD-127, part IV, Development and Properties of Uranium BaseAlloys and Corrosion Resistant in High Temperature WaterRadiationStability of Uranium Base Alloys, May 1957, pages 25, 39.

1. A MEMBER SUITABLE FOR USE AS A FUEL ELEMENT IN A NUCLEAR REACTORCOMPRISING AN ALLOY WROUGHT TO THE SHAPE OF THE MEMBER, THE ALLOYCOMPOSED OF FROM 7% TO 20% BY WEIGHT OF NIOBIUM AND THE BALANCE BEINGURANIUM, THE ALLOY BEING SUBSTANTIALLY ENTIRELY IN THE GAMMA PHASE THEALLOY HAVING BEEN PRODUCE BY WORKING AN INGOT OF THE ALLOY INTO MEMBER,HOMOGENIZING THE MEMBER BY ANNEALING IT AT A TEMPERATURE IN THE GAMMAPHASE FIELD AND QUENCHING THE MEMBER TO RETAIN THE GAMMA PHASE STRUCTUREOF THE ALLOY
 3. IN THE METHOD OF PREPARING A MEMBER SUITABLE FOR USE ASA FUEL ELEMENT IN THE NUCLEAR REACTOR COMPRISING AN ALLOY COMPOSED OFFROM 7% TO 20% BY WEIGHT OF NIOBIUM AND THE BALANCE BEING URANIUM, THESTEPS COMPRISING WORKING AN INGOT OF THE ALLOY INTO THE MEMBER,HOMOGENIZING THE MEMBER BY ANNEALING AT A TEMPERATURE IN THE GAMMA PHASEFIELD AND QUENCHING THE MEMBER TO RETAIN THE GAMMA PHASE OF THE ALLOY.